The invention relates to a process for the treatment of materials in a closed drum. This process is capable of effecting physical and/or chemical interaction between hot gases and/or a liquid treatment agent and the particles or components of the materials to be processed.
In the pharmaceutical industry there is a constant need to mix, heat, wet, and dry powders and granulates. Depending on the type and quantity of the raw materials, these treatment steps often require different procedures. Some raw materials may only be agitated and/or heated gently when wetted or dried, while others tend to form lumps and/or a coating that tends to prevent complete drying. The uniform distribution of solvents or wetting agents over relatively small amounts of auxiliary or active substances is also frequently difficult to achieve.
Diverse methods and devices have been proposed for the purpose of solving these problems, which in many cases are still unsatisfactory. Hence, the granulation of powders still requires multi-step treatment, while the drying of larger batches often necessitates lengthy treatments. This also applies for drying operations in which hot air is drawn through or over a charge of wet solid, loose material and/or where the moisture is to be removed by the application of vacuum. The latter operation especially has the tendency, where particles of the charge are heated at the surface and start to dry out, to form a coating which impedes or even prevents further drying. On the other hand, by the application of vacuum, the mixture cools down rapidly because the vaporizing solvent extracts evaporation heat from the material, such that the drying is slowed down despite the applied vacuum.
According to the state of the art, the process material is generally mixed with a liquid treatment agent which may then be removed by means of heating and/or pressure reduction. Especially in the case of relatively small treating fractions, the exact amount and uniform distribution of the treatment agent can be difficult to achieve. The treatment agent is also not always easy to remove, especially when the process material can only be separated with a high energy or process effort.
In the case of polar solvents, including water, acting as treatment agents, due to the high chemical and physical reaction capability of these agents, these problems are especially pronounced so that the boundaries between "chemical" and "physical" bonds or binding energies in systems with relatively large surfaces often becomes indistinct.
It therefore often becomes necessary to forego the use of treating agents which are cheap and free of ecological problems, and to substitute another agent, or to dilute an agent. It is increasingly important to find energy-saving and environmentally safe measures in the processing of fine-particulate substances in various fields of technology, ranging from the processing of foodstuffs and detergents through the processing of medicaments to fertilizer and building material technologies.
Until the present time, the theoretical aspects of treatment and processing of powders have still not been mastered but rather conventional methods were used. For example, in the manufacture of medicines starch is still being used as a binding agent, where a certain amount of starch with substantial amounts of water is hydrolyzed for every charge. The water must then be removed in long drying operations. The agglomeration or granulation of the mass, which is then reduced to the desired grain size, is thereby achieved. For this reason, little progress has been made in the automation of these methods.
If however, in a process broken down into steps, care is taken to supply sufficient heat and/or treatment agent in the desired concentration during these steps, possibilities for improvement can be found.
If it is desired to granulate a carbohydrate such as lactose or sugar with an active substance, the powder mixture is treated with water. Some parts dissolve and after drying, coarse-grained structures are obtained. However, complete uniformity in the composition and grain size is by no means assured.
A specific example for the special problems of treating solid, loose process material is in the agglomeration of powdery mixtures, with or without a reactive boundary surface area, such as in the manufacture or processing of preparations capable of reaction in the presence of water for the production of CO.sub.2. Such materials are referred to below as "reactive products".
The manufacture of such reactive products is only one of many examples of the problems of treating fine-grained process material. Another example is the granulation of substances which are spray-dried or which are obtained by fine grinding, or by chemical surface reactions.
In the past, processes for drying having become known, e.g. according to European Pat. No. 80,199, U.S. Pat. No. 4,535,550 or German Pat. No. 2,942,325. They however have the disadvantage that for a prior coating, granulation or the like, it is not easy to distribute the coating or granulation material uniformly in the charge material.
It has also already been proposed to supply a vacuum to the agent into the drum to achieve the desired treatment before drying is resumed, such as taught by DE-C-1,060.093, German Pat. No. 2,218,729 or AAT-C-372.299, or to jet the agent into the drum, according to European Pat. No. 80,199. However, in this case the treatment agent concentrates at the point of initial contact with the particles and forms initial large clumps, it being difficult or in some cases impossible to subsequently distribute the treatment agent uniformly in the charge material.
In general, during the treatment according to the present invention, such as during granulation of an effervescent mixture, heated air is laden with water vapor which condenses on the product. Afterwards the vapor is shut off, with carbon dioxide formed, as well as water vapor being removed by hot air. Vacuum is maintained at between 600-900 mbar.
In European Pat. No. 151,782, the one-time treatment of a solid, loose material in a vacuum drum with hot air flow, eventually laden with water vapor, is disclosed with the hot air flow being conducted through the charge. However, several disadvantages are evident. In the mixture, the nozzles can clog with moist, fine-grained material therein.
To achieve the treatment in one operation, the temperature and/or concentration of the vapor must be relatively high. However, that results in local overheating and/or a non-uniform condensation.
To a certain extent disadvantageous, is the one step process described in DE-B-1,060.093, in which heating is not necessarily done, but for the purpose of granulation of a powder, a vaporous solvent is taken in a single time and then removed after condensation by vacuum.
In GB-A-2,301,570, a drying process was described in which liquid or pasty products are subjected intermittently to a vacuum and then to a heated, dry gas flow. However, in this process even with products containing relatively little moisture such as yeast or onions, long drying times are required. This is associated with the fact that the air flow is conducted in a circuit and is freed of solvent vapor via an absorber bed, involving a considerable pressure drop. Therefore, the intake phase takes too long while the air flow brings along little heat to apply to the material which has been cooled down too much by the applied vacuum. To overcome this problem, a heat exchanger is used for preheating the air although even this has a certain resistance to flow. If this resistance is now overcome in the conventional manner, a relatively high vacuum will occur in the drum, resulting in severe cooling of the material. By virtue of hot air being applied by vacuum, there is a reduction in temperature of the particles caused by the evaporation of the moisture to be removed.
The usual treatment operations between a powdery particulate material and a liquid, such as a solvent, are broken down into individual steps which are easier to monitor, control and programmable. Up to this time, this did not appear possible in connection with such treatment operations because liquids such as binding agent solutions, or even water, are still too viscous to enable uniform distribution to be achieved in particle sizes between 10 and 100 microns. To date, a surplus of solvent had to be added to the solvent to achieve wetting of all particles.